An optical fiber and a bundle of optical fibers are variously modified in accordance with the conditions under which they are used. However, a tension member must be used with an optical fiber to assure a high strength in some cases. When water permeates an optical fiber cable, its strength may be degraded. When an optical fiber cable is to be installed in the bottom of a sea or the bottom of the water, in order to assure a sufficient installation tension and a high water resistance, an optical fiber cable must be used in a jacket structure in which an optical fiber cable is covered with a thin metal tube.
When an optical fiber cable having a small diameter is to be conventionally covered with a metal tube, an optical fiber is inserted into a metal tube having a longitudinal gap, and this gap is bonded by soldering.
According to this method, however, heat generated during bonding of the gap of the metal tube is applied to the optical fiber cable for a relatively long period of time to cause thermal damage.
In order to prevent this optical fiber cable from thermal damage, there is provided an apparatus and method of welding abutment portions of a metal tube with a focused laser beam to continuously manufacture a metal tube covered optical fiber cable, as disclosed in Published Unexamined Japanese Patent Application No. 64-35514.
This apparatus for manufacturing the metal tube armored optical fiber cable forms a continuously fed flat metal strip into a metal tube having a longitudinal gap at a top portion. A guide tube is inserted into the metal tube through this gap of the metal tube, and an optical fiber is guided into the metal tube through the guide tube. After the gap of the metal tube having received this optical fiber is closed, the metal tube is supplied to a laser welding unit.
The laser welding unit causes a guide roller to align the abutment edge portions of the top portion of the metal tube to each other. A laser beam having a focal point at a position outside the range of the abutment portions is radiated to weld the abutment portions. Since the laser beam is focused outside the range of the abutment portions, the abutment portions can be welded without protecting the optical fiber with a heat-shielding member.
This metal tube containing the optical fiber cable is drawn to have a predetermined outer diameter, and the drawn tube is continuously wound around a capstan.
During drawing of this metal tube, an inert gas is supplied to the guide tube to carry the optical fiber cable by the viscosity resistance of the gas. While the metal tube is kept engaged with the capstan, the optical fiber cable is blown outward against the inner surface of the metal tube, so that the length of the optical fiber cable is set larger than that of the metal tube. The optical fiber cable is not kept taut to prevent the optical fiber cable from stress caused by an installation tension or the like.
In order to protect the optical fiber cable from water entering from a hole formed in a damaged metal tube, a gel is injected inside the metal tube. More specifically, after the optical fiber cable is blown outward against the inner surface of the metal tube by the inert gas at the capstan, the gel is injected from a gel guide tube different from the guide tube for guiding the optical fiber cable.
In the above conventional apparatus for manufacturing a metal tube armored optical fiber cable, any heat-shielding member for protecting the optical fiber is not used at the time of welding of the abutment portions of the metal tube, and the position of the guide tube for guiding the optical fiber cable is indeterminate, for this reason, the guide tube is located near the abutment portions of the metal tube. Therefore heat generated during laser welding is undesirably applied to the optical fiber cable to thermally damage the optical fiber cable. For example, when a temperature of the optical fiber cable near the abutment portions during laser welding is measured, a temperature near the optical fiber cable is increased to at least about 600.degree. C. For this reason, a fine crystal nucleus is formed and is grown to undesirably cause a devitrification phenomenon in which a scattering loss is increased.
Sputter components are produced during welding. When the guide tube is close to the abutment portions, the sputter components deposited on the guide tube tend to be brought into contact with a rear bead of the welded portion. When the sputter components are brought into contact with the rear surface of the welded portion, the welded portion is thermally unbalanced to cause incomplete connection. In order to prevent this incomplete welding caused by the sputter components, when production of the rear bead portion as a sputter source is suppressed, a nonwelded portion is undesirably formed. When the guide tube is kept in contact with the abutment portions, the same phenomenon as in incomplete welding caused by the sputter components occurs. For these reasons, it is undesirably impossible to perform a continuous manufacturing operation (operation for manufacturing a long cable) in practice.
When a radiation power density of a laser beam is excessively high during laser welding, the metal tube is heated to a temperature exceeding its melting point and is partially evaporated or removed to form a hole in an irradiated portion. In order to prevent this, a laser beam is not conventionally focused within the range of the wall thickness of each abutment portion of the metal pipe, but is focused above the surface of the metal tube. When the laser beam is focused above the metal tube, the welded portion has a section in which the lower portion is sagged, so that a shrinkage hole and a crack tend to be formed inside the welded portion. In addition, an amount of sputter components is also increased, resulting in inconvenience.